The present invention relates to a laser device comprising a laser active medium, an optical resonator system defining an optical axis, exciting means for exciting said laser active medium and enabling a stimulated emission of radiation of said laser active medium, and cooling means having first and second cooling elements arranged in spaced opposing relationship with surfaces facing each other, wherein said laser active medium is provided between said cooling elements along said optical axis.
Laser devices as indicated above are generally known in the art. Basically, all kinds of laser devices have to deal with the problem of heat dissipation. For example, in a solid state rod laser device the surface of the rod which is subjected to excitation by a pumping light source such as a Xe lamp, is cooled and thus, a temperature gradient dependent on the cooling power and efficiency of the cooling means is generated. Thermal effects inside the rod strongly influence the optical properties of the laser active material such as the refractive index varying in dependence of the temperature distribution and birefringence, as well as of the further optical elements, namely the spherical resonator mirrors. It is difficult and costly to design a high quality laser system which is free of thermal degradation when the amount of heat generated during laser operation increases due to a desired increase of output power.
Consequently, laser devices were designed to improve both of the following aspects:
(i) increasing heat dissipation in order to achieve higher output power per unit volume of active laser material, and
(ii) minimising the influence of the temperature gradients on the optical properties of the laser device.
In the field of gas lasers using RF-excitation so called slab lasers or zig-zag lasers were developed. These lasers have opposing RF electrodes formed of rectangular plates having a reflective surface. These electrodes are arranged such that therebetween a volume is formed with the resulting gap being filled with a laser active gas. The distance between these electrodes is typically about 2 to 4 mm and may be increased up to 1 cm if an additional gas flow is provided, while the width of the gap in the direction perpendicular to the distance is in the order of several cm. The electrodes are cooled and thus, the heat is dissipated from the laser gas by conduction cooling with the surfaces of the electrodes. As a result a large cooling area and thus a large cooling power is established.
Simultaneously, this structure provides a temperature gradient which is substantially directed merely perpendicular to the surfaces of the electrodes. except for distortions at the lateral ends of the plates. This structure tends to cancel the effect of temperature gradients since the laser beam zigzags in the plane of temperature variation. However, the relatively large distance of the plates in the cm range results in a wide range of reflecting angles in this plane. Therefore, the resulting made of the radiation is no longer determined by the spherical resonator mirrors to be single mode but consequently the radiation is a multi mode one. However, many applications of laser devices require a fine focusing of the laser beam to achieve a high power density and thus, a multi mode beam is not desired.
Furthermore, so called xe2x80x9cwaveguidexe2x80x9d laser devices are known in the art wherein the radiation field in both of the transverse directions is confined by highly polished and highly reflective side walls. The radiation mode is completely determined by the waveguide cavity, whereas the resonator is merely composed of plane mirrors. In order to obtain single mode radiation from a waveguide laser the dimensions of the waveguide cavity are restricted to merely a few mm (2-4 mm). Waveguide lasers have excellent thermal properties, but the output power is low due to the small active volume.
U.S. Pat. No. 5,123,028 disposes a CO2 stab laser including a waveguide arrangement formed by a pair of spaced apart planar electrodes having opposed light reflecting surfaces. The confinement of the radiation in the plane parallel to the electrode surfaces is attained by a negative branch unstable resonator.
WO 95/2909 describes a CO2 slab waveguide laser wherein the light propagation path in plane parallel to the waveguide surfaces is folded by spherical mirrors.
GB 2276031 discloses a solid state laser device comprising a slab-shaped laser medium which has a pair of optically smooth surfaces. In the width dimension of the slab-shaped laser medium an unstable free space resonator is provided, whereas in the narrow spaced thickness direction of the slab medium confinement of the radiation is achieved by the optically smooth surfaces of the medium.
In U.S. Pat. No. 4,719,639 a laser device is disclosed, wherein by means of highly polished and highly reflective electrodes having a small distance of less than 5 mm in one transverse dimension waveguide conditions are created, while the other transverse dimension remains xe2x80x9copenxe2x80x9d, i.e. the optical cavity in this direction is confined by spherical resonator mirrors. An arrangement as mentioned in the above disclosure succeeds in increasing the active volume in comparison to the waveguide laser, while on the other hand, a single mode radiation can be obtained. However, the distance of the electrodes is limited to typically 2-3 mm. In addition the electrodes require a high optical quality as well as high parallelism.
In view of the above mentioned problems and disadvantages of the prior art it is therefore an object of the present invention to provide a laser device of a compact structure having increased optical power and outputting finely focusable radiation.
The above mentioned object is solved by a laser device comprising a laser active medium, an optical resonator system defining an optical axis exciting means for exciting said laser active medium and enabling a stimulated emission of radiation of said laser active medium, and cooling means having first and second cooling elements arranged in spaced opposing relationship with surfaces facing each other, wherein said laser active medium is provided between said cooling elements along said optical axis and the laser device is characterised in that an optical element is provided, arranged within the optical path formed by said optical resonator system and having a refractive power in a first plane along the optical axis and perpendicular to said surfaces, differing from a refractive power in a second plane along the optical axis and perpendicular to said first plane, wherein said refractory power in said first plane of said optical element is adjusted so as to prevent interaction of the lowest order radiation mode with the surfaces of said first (61) and second (62) cooling elements.
The term xe2x80x9crefractive powerxe2x80x9d refers to all kinds of optical elements, particularly to refractive, diffractive and reflective optical elements having the ability to collimate or disperse a light beam.
The term xe2x80x9calong the optical axisxe2x80x9d includes all arrangements of said planes parallel to the optical axis or the optical axis being within said planes.
xe2x80x9cA plane along the optical axis perpendicular to said surfacesxe2x80x9d represents a plurality of planes, which are parallel if the surfaces of the cooling elements are even and include a certain angular range if the surfaces are bent. The above definition may also include the case in which the xe2x80x9cplanesxe2x80x9d are no longer even but comprise a certain curvature according to the spaced relationship of the cooling elements, for instance if the surfaces of the cooling elements are cylindrical with non-coinciding centres of curvature.
The definition of radiation mode used in this application is, with reference to the related art, to be understood in the following way:
In case of a free space propagation of the laser beam the TEM∞ mode constitutes the so-called fundamental or lowest order mode whereas the all other modes are indicated as higher order modes.
For a laser device having a square-shaped waveguide cavity the EH11 mode is considered as the fundamental or lowest order mode and all other modes are treated as the higher order modes.
For a laser device having a slab-like waveguide cavity the radiation mode with a single maximum in its distribution of the transverse electric field in the non-free-space propagation direction is considered as the fundamental mode or the lowest order mode.
According to the present invention, in the laser device as defined above, the radiation generated within the active volume, i.e. within the laser active medium, is collimated or dispersed, respectively by an optical element in the plane which is perpendicular to surfaces of the first and second elements of the cooling means defining the location of the laser active medium, so as to confine, in combination with the arrangement of the spherical resonator mirrors, i.e. distances of the resonator mirrors, the curvatures thereof and the curvature of the optical element, the radiation field in this plane. The restriction of the radiation""s angular range in the plane perpendicular to the surfaces of the cooling elements allows a larger separation of the cooling elements which were previously used for confining the laser radiation while maintaining an operation of the laser device in a single transverse mode.
The laser device according to the present invention unifies the advantages of small dimensions with increased output power. Reduced manufacturing costs result from the fact that the surfaces of the cooling elements need not be of high optical quality and the number of peripheral devices for cooling the laser active medium is reduced compared to a prior art laser device. In addition, the amount of required maintenance during the operation of a laser device according to present invention is low as the thermal stress of the various components of the laser device is reduced due to the increased cooling efficiency. Furthermore, misalignment of the cooling elements due to thermal expansion merely insignificantly affects the operation properties of the laser device since the distance of the cooling elements and their parallelism are not critical parameters. A further advantage is an increased lifetime resulting from the improved cooling efficiency.
In a further preferred embodiment of the present invention aperture means are disposed in the optical path. These aperture means can be realised, for example, by diaphragms which restrict the optically effective distance of the surfaces of the cooling elements. Preferably, the mutual arrangement of said optical element, optical resonator means and aperture means is optimised so as to minimise the radiation losses at said aperture means. The process of optimising the laser device in order to select a single mode operation in the plane in which the optical element controls the confinement of the radiation is considerably facilitated by these aperture means.
In a preferred embodiment of the present invention the optical element and the resonator system in combination form a stable resonator in the plane perpendicular to the surfaces of said cooling means. As a result, the laser beam never xe2x80x9ctouchesxe2x80x9d the surfaces of the cooling elements in this arrangement.
Advantageously, the optical axis of the laser device is divided into portions forming an angle in respect to each other, thereby reducing the overall longitudinal dimension of the laser device while simultaneously maintaining a large optically active volume.
Preferably, the optical element comprises a reflective surface in order to divide the optical axis in a first and at least one further portion, thereby reducing the number of required optical components in the laser device.
The optical element for collimating and dispersing, respectively, is preferably a cylindrical mirror which can be manufactured at relatively low cost.
Preferably, the curvatures of the mirrors of the optical resonator system are formed such that they can simultaneously serve as the optical element, thereby reducing the number of required optical components. A mirror of such design would be prererably bi-cylindrical.
In a further preferred embodiment of the present invention the distance of the surfaces varies along the optical axis correspondingly to a beam profile in this section. By this measure of adaption of the shape of the cooling elements to the beam diameter variation along the optical axis. Both the cooling efficiency and the optical power are optimised since the distance of the surfaces always remains at its minimally required value.
Preferably, the first and second elements of the cooling means are made of an electrically conductive material. This provides the possibility to use the elements as electrodes. Furthermore, a high electrical conductivity usually also implies a high thermal conductivity, which means a uniform temperature distribution over the surfaces of the elements and thus, generating a one-dimensional temperature gradient.
In a preferred embodiment of the present invention said exciting means is a radio frequency source including a power matching circuitry, and said radio frequency source is electrically connected to said first and second elements of said cooling means. In this manner, since the elements of the cooling means have a double function, the dimensions of the laser device can further be reduced.
The laser active medium is preferably a laser gas. In conjunction with the elements of the cooling means which are also serving as RF electrodes an effective cooling of the laser gas by means of conduction cooling is accomplished and thus, a powerful gas laser having a large laser active volume within the laser gas compared to the prior art gas lasers is created.
In a preferred embodiment of the present invention said gas comprises the components: CO2, N2, He, Xe. The usage of the additional component Xe results in an increasing output power of about 20-30% due to the modification of the velocity spectrum of the electrons exciting the molecule oscillations in CO2 and N2.
Preferably, the cooling elements comprise passage ways in order to allow a gas flow for increasing the cooling capacity and replacing the degraded CO2 molecules.
Advantageously, said optical resonator system is a negative branch unstable resonator system. The negative branch unstable resonator is an unstable resonator system comprising a focus between the resonator mirrors. In this case, the optical element has to have a collimating effect in order to confine the radiation field. The sensitivity to misalignment of this type of resonator is much smaller than that of a positive branch unstable resonator or of a stable resonator. Thus, the influence of thermal expansion and mechanical stress resulting in misalignment during operation is less important compared to the prior art devices.
In the laser device of the present invention the surface distance of a single device can be selected as a value in the range from 2 mm to 15 mm. Even for a value of 2 mm the propagating laser light never touches the surfaces of the cooling elements. Moreover, by employing an additional slow-flow gas exchange across the cooling elements the output power of the laser device may be significantly increased (several times of the output power without a gas flow).
In a further preferred embodiment of the present invention the optical axis is divided into several portions, the several portions not being in a common plane. Various parts of a laser device containing an active volume may thus be arranged in a xe2x80x9c3-dimensionalxe2x80x9d structure resulting in a high output power device with small overall dimensions.
In a further preferred embodiment of the present invention, the cooling elements are formed to have plane surfaces facing each other. This results in a simple geometry of the active volume of the laser device and hence, in an easy-to-build optical element since the planes in which the optical element collimates or disperses, respectively are parallel. Thus the optical element can be provided, for example, as a cylindrical mirror.
Further preferred embodiments follow from the dependent claims.